Metal oxide-hydrogen batteries, such as nickel oxide - hydrogen batteries, have seen use in aerospace applications and can be used in aircraft starting because they are rechargeable and provide a uniform output during the entire discharge cycle.
In the typical nickel oxide-hydrogen battery, the positive electrodes are generally in the form of flat, porous, sintered, nickel plaques impregnated with nickel hydroxide, while the negative electrodes are formed of a fine nickel mesh screen having a catalyst, such as platinum black, bonded to one surface of the screen through a hydrophobic polymeric material. On discharge of the battery, hydrogen gas contained within the vessel diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates and becomes dissociated by the catalyst to the monoatomic form. The monoatomic hydrogen is ionized and combines with hydroxy ions to form water, with an electron being released in the process of forming each hydrogen ion. In addition, hydroxy ions are formed at the positive electrode by the reaction of water with the available oxygen content of the nickel oxide. As a result of these reactions, an electron current is produced in the exterior circuit. On recharging through application of an outside electron source, the reaction is reversed, causing the regeneration of hydrogen gas at the negative electrode and the re-oxidation of the nickel hydroxide at the positive electrode.
Due to the substantial gas pressures that are involved, the nickel oxide-hydrogen battery is contained within an outer pressure vessel. At start up, hydrogen gas is charged or filled into the pressure vessel at pressures as much as 700 psi. After filling, the vessel is sealed. During discharge, as previously noted, hydrogen gas will diffuse into the battery cells, while on recharging, hydrogen gas generated within the cell is discharged outwardly into the vessel.
Nickel oxide-hydrogen batteries have a relatively high rate of self-discharge during standby periods. For example, a typical nickel oxide-hydrogen battery will self-discharge and may lose approximately 15% of its service life in three days. This is compared with a conventional automotive battery which on self-discharge may lose approximately 1.5% of its service life within 30 days. Because of the relatively high self-discharging rates, a nickel oxide-hydrogen battery requires frequent recharging cycles so that the battery is in an operative condition at all times.
In aircraft use, the nickel oxide-hydrogen battery has particular use in restarting the engine when the aircraft is in flight, due to a flame-out or other engine stoppage. In satellite use, the battery is employed for performing certain working functions. Therefore, it is important that the metal oxide-hydrogen battery not be allowed to self-discharge so it can perform these functions at any time.